There is known a rear wheel toe angle controlling device configured to change the toe angles of right and left rear wheels individually by extending and retracting linear displacement actuators such as hydraulic cylinders mounted to parts of the vehicle body where lateral links or trailing links of a suspension device supporting the right and left rear wheels are joined (see Japanese patent laid open publication (kokai) No. 09-30438). Such rear wheel toe angle controlling device can steer right and left rear wheels in a same phase relationship and control their steering angle.
Conventional rear wheel steering controllers, including the one disclosed in the above Japanese patent laid open publication, are configured to control the rear wheel steering angle according to the turning behavior of the vehicle and thus improve vehicle stability and responsiveness. In such a rear wheel steering angle control, it is a common practice to determine a control target value of the rear wheel steering angle according to the front wheel steering angle, lateral acceleration, and/or yaw rate. According to one of such known control mechanisms, the rear wheel steering angle is controlled such that the slip angle becomes 0 (See Japanese Patent No. 3,179,271). The term “slip angle” refers to the angle between the travel direction to which the vehicle body is headed and the direction in which the vehicle is actually traveling when the vehicle is cornering.
A steady-state value of a vehicle slip angle β of a vehicle where only front wheels are steered can be obtained from the following equation:β={1−(m/L)·[Lf/(Lr·kr)]·V2}/(1+A·V2)·(δf·Lr/L)  (1)wherein, m: vehicle mass, L: wheelbase, Lf and Lr: distance from gravitational center, δf: front wheel steering angle, A: stability factor, V: vehicle velocity, kr: rear wheel cornering power.
In addition, as is obvious from FIG. 7 which shows the relation between (vehicle slip angle β/front wheel steering angle δF) and vehicle velocity V of vehicles having understeer, neutral steer and oversteer properties, the slip angle β has a positive value when the vehicle velocity is 0 and changes toward a negative value as the vehicle velocity increases, regardless of the steer property. This means that a vehicle turning with a low velocity tends to head outwardly from a regular turning radius, while a vehicle turning with a high velocity tends to head inwardly into a regular turning radius. As is obvious from equation (1) and FIG. 7, since the slip angle changes according to the vehicle velocity, regardless of the steer property, when the rear wheel steering angle is variably controlled such that the slip angle becomes 0 regardless of the vehicle velocity, a vehicle operator who is used to drive vehicles where only front wheels are steered will feel that the vehicle turns more than intended. In addition, when the vehicle accelerates or when there is a sudden change in road condition, the conventional rear wheel steering angle controlling mechanisms are not able to properly deal with these changes, and this may cause a discomfort to the vehicle operator.
The present invention was conceived in view of such problems of the prior art and its main object is to provide a rear wheel steering angle controlling device which can prevent the vehicle operator from experiencing a discomfort regarding the vehicle handling and improve vehicle stability and yaw responsiveness.